1. Field of the Invention
The present invention relates to a low phase noise differential LC tank voltage controlled oscillator (VCO) with current negative feedback, in which a phase noise characteristic is improved by mounting a current negative feedback circuit on a typical differential emitter-degeneration VCO.
2. Discussion of Related Art
A conventional differential LC tank cross-coupled pair VCO when combined with a phase-locked loop constitutes a frequency synthesizer, and is widely employed in a wireless transceiver integrated circuit. The differential LC tank cross-coupled pair VCO has an excellent noise characteristic compared with other oscillators due to a filtering function of an LC tank 100, however further reduction of noise is required.
Accordingly, the differential LC tank VCO increases a Q factor of an inductor in the LC tank 100 to reduce phase noise, or uses an LC filter to eliminate noise from a current source. Since phase noise in the differential LC tank VCO is caused largely by up-conversion of 1/f noise and thermal noise of a cross-coupled pair, efforts have been made to thoroughly reduce the influence of 1/f noise and thermal noise using a low-frequency negative feedback circuit. Recently, a differential emitter-degeneration cross-coupled pair VCO has been developed in order to upwardly adjust an oscillation frequency of the conventional differential LC tank cross-coupled pair VCO, reduce phase noise, and expand a frequency modulation range.
FIG. 1 illustrates a conventional differential capacitive-degeneration LC tank VCO 10. As illustrated in the VCO circuit 10 of FIG. 1, when a resistor 121 or a capacitor CP 125 is connected to an emitter of an oscillation transistor 111, a negative resistance range moves to a higher frequency. Accordingly, at the higher frequency, the VCO oscillates with larger amplitude and has an expanded frequency modulation range and reduced phase noise. Input impedance in a cross-coupled pair of the capacitive-degeneration LC tank VCO 10 is given by Equation 1.
                              Z          in                =                              -            2                    ⁢                      (                                          Z                E                            +                              1                                  g                  m                                            +                              s                ⁢                                                                            C                      π                                        ⁢                                          r                      b                                                                            g                    m                                                                        )                                              [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, ZE=RE//(1/sCP), CP denotes base-emitter capacitance, rb denotes base resistance, and gm denotes transconductance. As shown in Equation 1, negative resistance of the input impedance seen from collector nodes of oscillation transistors 111 and 112 is increased by parallel connection of emitter resistors 121 and 122 and capacitors 125 and 126. And, the third term in Equation 1 has an inductance character, which shows that the frequency modulation range is also widened.
In the capacitive-degeneration LC tank VCO 10 of FIG. 1, Vnf denotes a low-frequency noise source corresponding to 1/f noise and thermal noise in a base of each transistor, Vnbe denotes base-emitter voltage of a cross-coupled pair 111 and 112 induced by the low-frequency noise source, Intail denotes tail current noise, IE denotes emitter current, ΔIE denotes a range of fluctuation in the emitter current due to low-frequency noise, Vam denotes a low-frequency amplitude modulation (AM) signal of oscillation frequency, that is, carrier frequency, due to low-frequency noise, Vcm denotes a common-mode level, and ΔVcm denotes a range of fluctuation in the common-mode level due to low-frequency noise.
Elements affecting phase noise in the differential LC tank VCO 10 of FIG. 1 are the Q-factor of an inductor, low-frequency noise of a cross-coupled pair transistor, and low- and high-frequency noise of current source transistors 127 and 128. In the differential LC tank VCO 10 of FIG. 1, the base-emitter voltage Vnbe is induced by the low-frequency noise source Vnf at bases of the oscillation transistors 111 and 112, and the emitter current changes by as much as ΔIE due to the base-emitter voltage Vnbe. After the low-frequency amplitude modulation of a carrier signal by the change in emitter current, a mean capacitance value of a varactor changes due to the amplitude modulation signal Vam, and the carrier frequency fluctuates, thereby generating jitter, which contributes to phase noise.
Also, the IE is modulated by low-frequency noise of tail current, thereby causing Vcm to fluctuate by as much as ΔVcm. This fluctuation in turn causes variation in varactor bias voltage, which causes the carrier signal to fluctuate, thereby generating jitter, which contributes to phase noise.
Also, in the differential LC tank VCO 10 of FIG. 1, the base-emitter voltage Vnbe of the oscillation transistors 111 and 112 induced by the low-frequency noise source and the carrier signal get mixed due to nonlinear characteristics of the oscillation transistors 111 and 112, and thus the mixed noise contributes to noise around the carrier signal.
In the differential LC tank VCO 10 of FIG. 1, low- and high-frequency noise of the tail current induced by low-frequency noise sources and the carrier signal become mixed due to nonlinear characteristics of the oscillation transistors 111 and 112, and the mixed noise contributes to noise around the carrier signal.